Veteran Volkswagen Group engineer Wolfgang Hatz has been released after nine months in jail after providing $3.5 million in bail money, German prosecutors have confirmed.

He was arrested last September during investigations into the VW Group's diesel scandal. Although he is yet to be charged, the prosecutors in accordance with German law decided to place him in jail because he refused to give testimony, Handelsblatt reported last week.

Hatz was serving as R&D chief at Porsche at the time of his arrest. Prior to the role he was head of powertrain development at the Audi and Volkswagen brands.

Hatz's release has come at the same time as the arrest of Audi CEO Rupert Stadler by German authorities over concerns he could suppress evidence during the ongoing diesel scandal investigations. Prosecutors claim that a wiretapped conversation shows evidence of Stadler attempting to organize a meeting to tamper with evidence.

Unlike Hatz, Stadler is willing to give testimony, a spokesman for the prosecutor’s office told Handelsblatt.

The two join 18 other current and former VW Group execs under investigation by German authorities regarding the diesel scandal, which so far has cost the automaker $26 billion in damages.

Here in the United States, two Volkswagen executives have been sentenced for their involvement in the scandal. The first was James Liang who last August was sentenced to 40 months in jail. The second was Oliver Schmidt who last December was sentenced to seven years in jail.